Incidence of bubbles on samples cast in a phosphate-bonded investment

Abstract

P hosphate-bonded investments are widely used to produce castings of high-fusing alloys for both ceramometal and all-metal dental restorations. These investments are generally stronger, more refractory, and can be manipulated for a greater degree of mold expansion than gypsum-bonded investments. The investments are primarily composed of 75% to 90% silica refractory in the form of quartz or cristobalite, which is initially bound together by magnesium and ammonium magnesium phosphate formed by reaction of magnesium oxide with monoammonium phosphate in water. High temperature burnout results in loss of water, crystallization, recrystallization of magnesium phosphates and formation of silicophosphate glasses, which give the mold high strength at casting temperatures.“’ An unfortunate sideeffect of the initial setting reaction is the liberation of gas (presumably ammonia) bubbles, which often adhere to the surface of the wax pattern and result in positively cast nodules on the surface of the casting (Fig. 1). Such nodules interfere with the fit of the casting and must be removed before the casting is clinically serviceable. This is time-consuming and troublesome for the dental technician and may compromise the fit of the casting if excessive internal grinding is required. It was the purpose of this study to examine the effect of a number of investing variables on the incidence of bubbles cast to wax patterns invested in a phosphatebonded investment. The following investing variables were examined. 1. Liquid/powder ratios 2. Mixing time under vacuum 3. Low speed (hand) vs. high speed (machine) mixing 4. Special liquid/water ratios for a fixed total L/P ratio 5. Use of a debubblizer